import torch
import torch.nn.functional as F
from op_flow.utils import bilinear_sampler, coords_grid

try:
    import alt_cuda_corr
except:
    # alt_cuda_corr is not compiled
    pass


class CorrBlock:
    def __init__(self, fmap1, fmap2, num_levels=4, radius=4):
        self.num_levels = num_levels
        self.radius = radius
        self.corr_pyramid = []

        # all pairs correlation
        corr = CorrBlock.corr(fmap1, fmap2)#[batch, ht, wd, 1, ht, wd]#ht,w

        batch, h1, w1, dim, h2, w2 = corr.shape
        corr = corr.reshape(batch*h1*w1, dim, h2, w2)
        
        self.corr_pyramid.append(corr)
        for i in range(self.num_levels-1):
            corr = F.avg_pool2d(corr, 2, stride=2)
            self.corr_pyramid.append(corr)

    def __call__(self, coords):
        r = self.radius
        coords = coords.permute(0, 2, 3, 1)#[B,H,W,C],C=2  
        batch, h1, w1, _ = coords.shape

        out_pyramid = []
        for i in range(self.num_levels):
            corr = self.corr_pyramid[i]
            dx = torch.linspace(-r, r, 2*r+1, device=coords.device)#[2*r+1]
            dy = torch.linspace(-r, r, 2*r+1, device=coords.device)
            delta = torch.stack(torch.meshgrid(dy, dx), axis=-1)#[2*r+1,2*r+1,2] [9,9,2] 

            centroid_lvl = coords.reshape(batch*h1*w1, 1, 1, 2) / 2**i #[B*H*W,1,1,2]
            delta_lvl = delta.view(1, 2*r+1, 2*r+1, 2)#[1,2*r+1,2*r+1,2]
            coords_lvl = centroid_lvl + delta_lvl#[B*H*W,2*r+1,2*r+1,2]

            corr = bilinear_sampler(corr, coords_lvl)
            corr = corr.view(batch, h1, w1, -1)#[B，H，W,dim*（2*r+1）*（2*r+1）]
            out_pyramid.append(corr)

        out = torch.cat(out_pyramid, dim=-1)#[B，H，W,dim*（2*r+1）*（2*r+1）*4]
        return out.permute(0, 3, 1, 2).contiguous().float()

    @staticmethod
    def corr(fmap1, fmap2):
        batch, dim, ht, wd = fmap1.shape
        fmap1 = fmap1.view(batch, dim, ht*wd)
        fmap2 = fmap2.view(batch, dim, ht*wd) 
        
        corr = torch.matmul(fmap1.transpose(1,2), fmap2)
        corr = corr.view(batch, ht, wd, 1, ht, wd)
        return corr  / torch.sqrt(torch.tensor(dim).float())#[batch, ht, wd, 1, ht, wd]


class AlternateCorrBlock:
    def __init__(self, fmap1, fmap2, num_levels=4, radius=4):
        self.num_levels = num_levels
        self.radius = radius

        self.pyramid = [(fmap1, fmap2)]
        for i in range(self.num_levels):
            fmap1 = F.avg_pool2d(fmap1, 2, stride=2)
            fmap2 = F.avg_pool2d(fmap2, 2, stride=2)
            self.pyramid.append((fmap1, fmap2))

    def __call__(self, coords):
        coords = coords.permute(0, 2, 3, 1)
        B, H, W, _ = coords.shape
        dim = self.pyramid[0][0].shape[1]

        corr_list = []
        for i in range(self.num_levels):
            r = self.radius
            fmap1_i = self.pyramid[0][0].permute(0, 2, 3, 1).contiguous()
            fmap2_i = self.pyramid[i][1].permute(0, 2, 3, 1).contiguous()

            coords_i = (coords / 2**i).reshape(B, 1, H, W, 2).contiguous()
            corr, = alt_cuda_corr.forward(fmap1_i, fmap2_i, coords_i, r)
            corr_list.append(corr.squeeze(1))

        corr = torch.stack(corr_list, dim=1)
        corr = corr.reshape(B, -1, H, W)
        return corr / torch.sqrt(torch.tensor(dim).float())
